At first, an investigation into her complaint revealed that an edge-case bug—another way of saying a rare occurrence—had bumped her card balance over the limit. Many companies would dismiss such cases as outliers, and thus not worthy of attention, on the assumption that they rarely happen and are too expensive to fix. At Amazon, such cases were regularly attended to because they would happen again and because the investigation often revealed adjacent problems that needed to be solved. What at first looked to be just an edge case turned out to be more significant. The bug had caused problems in
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